Method of estimating vehicle velocity and method of and system for controlling brakes

ABSTRACT

A method of and an apparatus for estimating a vehicle velocity, which are suitable for use in a vehicle. Velocities of drive wheels and follower wheels of the vehicle are determined and the fastest one of the so-determined velocities is selected. The velocity of the vehicle is estimated based on the fastest wheel velocity thus selected. The above respective processes are repeatedly executed at given time intervals. Thus, when the estimated vehicle velocity is faster than each of the velocities of the follower wheels and lower than the fastest wheel velocity, the estimated vehicle velocity determined immediately before the above estimating process is set as a desired estimated vehicle velocity. Accordingly, a high-accuracy estimated vehicle velocity corresponding to an actual vehicle velocity can be obtained, thereby making it possible to control brakes and driving forces with high accuracy using the estimated vehicle velocity.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of and an apparatus forestimating a vehicle velocity, which are suitable for use in a vehiclehaving drive wheels and follower wheels, and to a method of and a systemfor controlling brakes, wherein when a braking force to be applied toeach of the brakes is estimated from a wheel slip ratio and a wheelacceleration/deceleration so as to control each brake or when braking ischanged from an antilock brake system (ABS) mode to normal braking, theoptimum brake pressure increasing rate can be set upon increase in thebrake pressure and the braking force can be controlled based on theoptimum brake pressure increasing rate, thereby making it possible toensure a feeling of satisfactory control.

2. Description of the Related Art

In a vehicle such as a motorcar, a motorcycle or the like, a so-calledbrake control system is used in which a speed or velocity of each wheelplaced under braking is compared with a vehicle speed and controlling ofbrakes is effected based on the result of comparison. In the brakecontrol system, a slip ratio is determined from the wheel velocity andthe vehicle velocity. When the slip ratio reaches a target slip ratio orabove, the slip ratio is reduced by decreasing brake hydraulic pressure,thereby producing the optimum braking force.

Further, a driving force control apparatus is known which controls adriving force of an engine by adjusting the ignition timing of theengine upon a vehicle rapid start or depending on a variation in afriction coefficient of a road surface, for example. Even in the case ofthe driving force control apparatus, the wheel velocity and the vehiclevelocity are used as data.

Now, the wheel velocity, i.e., the rotational speed of each wheel can bedirectly detected by a sensor. It is however difficult to directlydetect the vehicle velocity by a sensor. It is also next to impossibleto detect the velocity of a vehicle such as a motorcycle whose weightand size are greatly restricted to accommodate the sensor therein.Accordingly, a method is normally used which estimates the vehiclevelocity from the wheel velocity.

However, there is often a situation in which an estimated vehiclevelocity is set to be larger or smaller an actual vehicle velocity underthe conditions in which each wheel slips against the road surface. Inthis case, the brake control system or the driving force controlapparatus tend to effect unsuitable control.

A brake control system is known, in which a slip rat of each wheelagainst a road surface is computed from the speed or velocity of arunning vehicle and the rotational speed of each wheel and the optimumbraking force is applied to the vehicle based on the computed slipratio. As an example of such a system, a control logic circuit of whichis shown in FIG. 1. In FIG. 1, each of an inlet valve and an outletvalve is a hydraulic control valve for controlling hydraulic pressureapplied to a caliper cylinder (hereinafter called "caliper pressure") tooperate a pair of calipers which hold each brake disk therebetween.

In the same drawing, each of λ₁, λ₂ and λ₃ represents a slip ratio ofeach wheel against the road surface. They have a relationship of λ₁ <λ₂<λ₃. Each of α₁, α₂ and α₃ represents a wheel acceleration and each of-α₁ and -α₂ represents a wheel deceleration. These values have arelationship of -α₂ <-α₁ <0<α₁ <α₂ <α₃. Now, a parameter represented by-α₁ and -α₂ is changed from "0" to "1" when each of the wheeldecelerations has reached a set value (threshold value) or less. Each ofparameters other than the parameter referred to above is changed from"0" to "1" when each deceleration has reached the threshold value ormore. In the case of the slip ratio, on the other hand, outputs appearon signal lines or conductors set by λ₁, λ₂ and λ₃ respectively when theslip ratio has reached each of given slip ratios (λ₁, λ₂ and λ₃) orabove (threshold value or above).

The ABS is provided with a modulator, and when the ABS is activated,control of the caliper pressure by the driver, is modified by theoperation of the modulator having the aforementioned inlet valve(normally closed to a pressurized fluid source) and aforementionedoutlet valve (normally open to a fluid exit), both controlled by theabove-described control logic circuit. The modulator increases ordecreases the caliper pressure, in response to changes in thepressurized fluid, regulated by such valves. Depending on the operationof the control logic circuit described above, three events may occur.When the inlet and outlet valves are not operated (i.e. their normalstate), the modulator releases pressurized fluid through the outputvalve, and increases caliper pressure, to increase braking up to apredetermined maximum. If only the outlet valve is operated (i.e. boththe inlet and outlet valves are closed), the modulator remains in aconstant state and likewise, the caliper pressure is kept constant. Ifboth the inlet and outlet valves are operated (i.e. opened and closedrespectively), then in response to the increased pressurized fluid, themodulator effects a decrease in caliper pressure to reduce braking. Theabove is summarized in FIG. 2.

Thus it is seen that the brake control is effected by setting athreshold value for each of the slip ratio λ and theacceleration/deceleration α, and determining whether the actual state ofeach wheel (i.e. λ and α), is at their respective threshold value orabove (less). It is thus necessary to set processing times as short aspossible to improve the operating speed of an actuator which executesthe process referred to above. However, there are limitations on theoperating speed of the actuator, and an improvement is actuallydifficult to achieve.

In another prior art system, the modulator comprises an input hydraulicchamber which communicates with a master cylinder, for converting abrake operating instruction generated by the operation of a lever by adriver or the depression of a pedal by the driver into hydraulicpressure or power, an output hydraulic chamber which communicates with acaliper cylinder, for applying a braking force to a brake disk of eachwheel (hereinafter called as "caliper force"), a cut valve for causingthe input hydraulic chamber to communicate with "the output hydraulicchamber and for cutting off the communication between the input andoutput hydraulic chambers, an expander piston disposed on the outputhydraulic chamber side for closing the cut valve upon antilock brakingand for increasing the volume of the output hydraulic chamber so as toreduce the hydraulic pressure or power, and a crank member held inabutment against the expander piston and rotatable by a rotative drivesource.

In the modulator, the caliper pressure is reduced by displacing theexpander piston so as to increase the volume of the output hydraulicchamber to avoid a locked state of each wheel upon braking. When therisk of the locked state is avoided, the expander piston is displaced toopen the cut valve, thereby affecting normal braking.

In the prior art, however, when the braking is changed from the antilockbraking to the normal braking, caliper pressure P_(c) is abruptly raisedtoward master pressure P_(m) developed in the master cylinder at themaximum pressure increasing rate as indicated by the broken line definedbetween Q and R in FIG. 3.

When a vehicle travels from a road surface having a low frictioncoefficient (hereinafter called a "low μ road") with respect to eachwheel to a road surface having a high friction coefficient (hereinaftercalled a "high μ road") with respect to each wheel while the antilockcontrol is being effected during the braking of the vehicle, the frontwheel first comes across to the high μ road. Thus, a gripping force ofthe front wheel is raised so that the slip ratio is reduced, therebyenabling control for increasing the brake pressure. However, the rearwheel is still placed on the low μ road. Therefore, when the caliperpressure P_(c) applied to the front wheel is simply raised at themaximum pressure increasing rate, the braking forces of the front andrear wheels against the road greatly differ from each other. This tendsto hurt the control feeling. It is thus preferable to maintain thepressure increasing rate at a given value until the rear wheel reachesthe high μ road.

A modulator provided with a double-structure type cut valve having adual orifice defined therein is therefore known as has been disclosed inJapanese Patent Application Laid-Open Publication No. 49-15874 (whichcorresponds to U.S. Pat. No. 3,836,207). However, this modulator is alsoactuated by the pressure difference developed in hydraulic pressures,between the input hydraulic chamber and the output hydraulic chamber.Therefore, the pressure increasing rate is restricted and hence variouspressure increasing rates suitable for the conditions of the roadsurface or the state of braking cannot be realized.

SUMMARY OF THE INVENTION

It is a principal object of the present invention to provide a method ofand an apparatus for estimating a vehicle speed, which are suitable foruse in a vehicle, wherein a desired vehicle speed required to controlbrakes and brake driving forces or the like can be estimated easily andwith high accuracy.

It is another object of the present invention to provide a method of anda system for controlling brakes, wherein a braking force which isapplied to each of the brakes can be easily and accurately estimatedbased on a wheel acceleration/deceleration and a slip ratio, therebymaking it possible to effect the optimum brake control.

It is a further object of the present invention to provide a method ofand a system for controlling brakes, wherein satisfactory controlfeeling can be reliably achieved without regard to the conditions of aroad surface or the state of braking when an increase in a caliperpressure is effected.

It is a still further object of the present invention to provide amethod of estimating a vehicle velocity, which is suitable for use in avehicle, the method comprising the following steps: a first step ofdetermining velocities of drive wheels and follower wheels of thevehicle, a second step of selecting the fastest one of the velocitiesdetermined in the first step, a third step of estimating the velocity ofthe vehicle based on the fastest wheel velocity selected in the secondstep, and a fourth step of repeatedly executing the first through thirdsteps at given time intervals and setting the estimated vehicle velocitydetermined immediately before the third step as an estimated vehiclevelocity to be determined in the fourth step when the estimated vehiclevelocity determined in the third step is faster than the follower wheelvelocities determined in the first step and lower than the fastest wheelvelocity selected in the second step.

It is a still further object of the present invention to provide anapparatus for estimating a vehicle velocity, which is suitable for usein a vehicle. The apparatus comprises first wheel velocity detectingmeans for detecting velocities of drive wheels, second wheel velocitydetecting means for detecting velocities of follower wheels, wheelvelocity selecting means for selecting the fastest one of the respectivewheel velocities detected by the first and second wheel velocitydetecting means, vehicle velocity estimating means for estimating thevelocity of the vehicle based on the fastest wheel velocity selected bythe wheel velocity selecting means, vehicle velocity storing means forstoring therein the vehicle velocity estimated by the vehicle velocityestimating means, and comparing means for comparing the vehicle velocitystored in the vehicle velocity storing means and the follower wheelvelocities detected by the second wheel velocity detecting means. Thevehicle velocity estimating means is activated to estimate a desiredvehicle velocity supposing the amount of change of the estimated vehiclevelocity into the high-velocity side to be zero when it is determinedbased on the result of comparison by the comparing means that thevehicle velocity is faster than the velocities of the follower wheels.

It is a still further object of the present invention to provide amethod of controlling brakes, wherein the stability of running of avehicle and the state of braking applied to the vehicle are controlledby adjusting caliper pressure according to the state of running of thevehicle. The method comprises the steps of determining slip ratios ofwheels, determining accelerations and decelerations of the wheels, andestimating the amounts of increase and decrease in the caliper pressurefrom a membership function in which the determined slip ratios and thedetermined accelerations and decelerations are set as inputs.

It is a still further object of the present invention to provide amethod of controlling brakes, wherein the membership function is changeddepending on characteristics of tires fixed onto the wheels.

It is a still further object of the present invention to provide amethod of controlling brakes, wherein the membership function is changeddepending on the running stability of the vehicle.

It is a still further object of the present invention to provide asystem for controlling brakes, wherein the stability of running of avehicle and the state of braking applied to the vehicle are controlledby adjusting caliper pressure according to the state of running of thevehicle. The system comprises wheel acceleration/deceleration detectingmeans for detecting an acceleration and a deceleration of each wheel,slip ratio computing means for computing a slip ratio with respect tothe surface of a road traveled by each wheel, storing means for storinga table therein as information, the table including the amounts ofincrease and decrease in the caliper pressure, which have been set so asto correspond to the value of the detected acceleration/deceleration andthe value of the computed slip ratio, and caliper pressure controllingmeans for increasing and decreasing the caliper pressure according tothe amounts of increase and decrease in the caliper pressure, which havebeen set based on the table from the wheel acceleration/deceleration andthe slip ratio.

It is a still further object of the present invention to provide asystem for controlling brakes, wherein the table is set based onmembership functions corresponding to the wheelacceleration/deceleration and the slip ratio.

It is a still further object of the present invention to provide asystem for controlling brakes, wherein the table is set so as to bringthe value of the wheel acceleration/deceleration and the value of theslip ratio into high resolution in the vicinity of target values towhich the wheel acceleration/deceleration and the slip ratio convergeand so as to bring same into low resolution as the wheelacceleration/deceleration and the slip ratio are separated from theconvergent target values.

It is a still further object of the present invention to provide asystem for controlling brakes, wherein the table is set so as to bringthe values of the slip ratio and the acceleration/deceleration into thelow resolution along directions in which the absolute values of the slipratio and the acceleration/deceleration increase from 0 slip ratio and 0wheel acceleration/deceleration respectively.

It is a still further object of the present invention to provide amethod of controlling brakes, wherein caliper pressure is transmitted toa caliper cylinder from a master cylinder depending on an input suppliedby operating a brake lever or a brake pedal or the like, therebyeffecting normal braking for applying a braking force to each wheel, anda cut valve is displaced upward and downward by an expander pistonmovable in upward and downward directions by a driving means so as to beclosed, thereby cutting off the caliper cylinder from communicating withthe master cylinder and adjusting the volume of an output hydraulicchamber which communicates with the caliper cylinder so as to effectantilock braking for controlling the caliper pressure. The methodcomprises the steps of controlling the caliper pressure which is appliedto each wheel to thereby effect the antilock braking, and moving theexpander piston upward and downward upon the antilock braking so as torepeatedly open and close the cut valve at given time intervals, therebyincreasing the caliper pressure at a target pressure increasing rate.

It is a still further object of the present invention to provide amethod of controlling brakes, wherein caliper pressure is transmitted toa caliper cylinder from a master cylinder depending on an input suppliedby operating a brake lever or a brake pedal or the like, therebyeffecting normal braking for applying a braking force to each wheel, anda cut valve is displaced upward and downward by an expander pistonmovable in upward and downward directions by a driving means so as to beclosed, thereby cutting off the caliper cylinder from communicating withthe master cylinder and adjusting the volume of am output hydraulicchamber which communicates with the caliper cylinder so as to effectantilock braking for controlling the caliper pressure. The methodcomprises the following steps: a first step of detecting the state ofinput, a second step of detecting the state of a road surface, a thirdstep of setting the rate of increase in the caliper pressure at the timeof the antilock braking, based on the detected state of input and thedetected state of road surface, and a fourth step of displacing theexpander piston in accordance with the set pressure increasing rate soas to increase the caliper pressure.

It is a still further object of the present invention to provide amethod of controlling brakes, wherein the second step includes a processfor estimating the state of the road surface from a vehicleacceleration/deceleration.

The above and other objects, features and advantages of the presentinvention will become apparent from the following description and theappended claims, taken in conjunction with the accompanying drawings inwhich preferred embodiments of the present invention are shown by way ofillustrative example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a conventional control logic circuit;

FIG. 2 is a view for describing control executed with respect to theoutputs of the logic circuit shown in FIG. 1;

FIG. 3 is a view for describing a caliper pressure increasing rateemployed in a brake control method according to a prior art;

FIG. 4 is a block diagram showing the structure of a vehicle speedestimating apparatus according to the present invention, which issuitable for use in a vehicle;

FIG. 5 is a view for describing the relationship between a wheel speedselected by a wheel speed selecting circuit of the apparatus shown inFIG. 4 and an estimated vehicle speed determined from a basic computingprocess of an estimated vehicle speed computing circuit;

FIG. 6 is a view for describing the relationship between the speed of afollow-up wheel, the speed of a drive wheel, a selected wheel speed anda vehicle speed estimated and corrected based on these speeds, all ofwhich are determined by the apparatus shown in FIG. 4;

FIG. 7 is a view for describing the relationship between the speed ofthe follow-up wheel, the speed of the drive wheel, the selected wheelspeed and the vehicle speed estimated and corrected based on thesespeeds, all of which are determined by the apparatus shown in FIG. 4upon brake control;

FIG. 8 is a schematic view illustrating the structure of a system foreffecting a brake control method according to the present invention;

FIG. 9 is a schematic exterior view depicting a motorcycle in which thebrake control system shown in FIG. 8 is to be incorporated;

FIG. 10 is a view for describing a fuzzy map employed in the brakecontrol method according to the present invention;

FIG. 11 is a view for describing a membership function of a slip ratio;

FIG. 12 is a view for describing a membership function of anacceleration/deceleration;

FIG. 13 is a view for describing a membership function of caliperpressure;

FIG. 14 is a view for describing the relationship between a wheel speedand caliper pressure both employed in the present invention and thoseemployed in the prior art;

FIG. 15 is a view for describing characteristics of different tires;

FIG. 16 is a view for describing a membership function of a slip ratio,based on the characteristics of the different tires;

FIG. 17 is a view for describing a membership function of a slip ratio,based on characteristics of a vehicle having excellent running stabilityand normal vehicle characteristics;

FIG. 18 is a schematic view showing the structure of a brake controlsystem according to another embodiment of the present invention;

FIG. 19(A) and 19(B) are the views for describing a table employed inthe brake control system shown in FIG. 18;

FIG. 20 is a view for describing the relationship between a slip ratioand the number of addresses or an interval for setting the amount ofincrease or decrease in pressure, all of which are represented in thetable employed in the brake control system shown in FIG. 18;

FIG. 21 is a view for describing the relationship between a wheelacceleration/deceleration and the number of addresses or an interval forsetting the amount of increase or decrease in pressure, all of which arerepresented in the table employed in the brake control system shown inFIG. 18;

FIG. 22a is a view for describing the relationship between a slip ratioand the amount of increase and/or decrease in caliper pressure, whichare shown in the table employed in the brake control system shown inFIG. 18;

FIG. 22b is a view for describing the relationship between a slip ratioand the amount of increase and/or decrease in caliper pressure, the viewbeing illustrated as a comparative example of the view depicted in FIG.22a;

FIG. 23 is a view for describing a method of controlling brakes,according to the present invention;

FIG. 24 is a view for describing the comparison between a caliperpressure increasing rate employed in the prior art and that employed inthe present invention;

FIG. 25 is a schematic view illustrating the overall structure of abrake control system according to a further embodiment of the presentinvention, for performing a brake control method according to thepresent invention;

FIG. 26 is a view for describing the manner of operation of a cut valvemechanism employed in the brake control system shown in FIG. 25;

FIG. 27 is a view for describing the manner of another operation of thecut valve mechanism employed in the brake control system depicted inFIG. 25;

FIG. 28 is a view for describing the manner of a further operation ofthe cut valve mechanism employed in the brake control system shown inFIG. 25;

FIG. 29 is a flowchart for describing an overall control routine whichis executed in the brake control method according to the presentinvention;

FIG. 30 is a flowchart for describing vehicle deceleration control whichis executed in the brake control method according to the presentinvention;

FIG. 31 is a flowchart for describing breakthrough control which isexecuted in the brake control method according to the present invention;

FIG. 32 is a view showing the result of control effected under a high μroad by the brake control method according to the present invention;

FIG. 33 is a view illustrating the result of control effected under a μjump by the brake control method according to the present invention;

FIG. 34 is a view depicting the result of control on a repetitive inputby the brake control method according to the present invention;

FIG. 35 is a view for describing the setting of a target crank angle bythe brake control method according to the present invention; and

FIG. 36 is a view for describing an increasing rate of caliper pressurewhich has been controlled by the target crank angle shown in FIG. 35.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A method of and an apparatus for estimating a vehicle speed or velocity,according to the present invention, which are suitable for use in avehicle, will hereinafter be described in detail with reference to theaccompanying drawings in which preferred embodiments are shown by way ofillustrative example.

FIG. 4 is a block diagram showing a vehicle speed estimating apparatusaccording to the present embodiment. The vehicle speed estimatingapparatus comprises a follower wheel rotational speed sensor 1 fordetecting the rotational speed or velocity of each follower wheel (e.g.a front wheel of a motorcycle, i.e. a 2-wheeled automotive vehicle), adrive wheel rotational speed sensor 2 for detecting the rotational speedor velocity of each drive wheel (e.g. a rear wheel of the motorcycle),wheel speed computing circuits 3A, 3B for computing wheel speeds orvelocities based on signals outputted from the follower wheel rotationalspeed sensor 1 and the drive wheel rotational speed sensor 2respectively, a wheel speed selecting circuit (wheel speed selectingmeans) 4 for selecting the fastest wheel velocity from the wheelvelocities which have been computed by the wheel speed computingcircuits 3A, 3B, an estimated vehicle speed computing circuit (vehiclespeed estimating means) 5 for computing an estimated vehicle speed orvelocity based on the wheel velocity which has been selected by thewheel speed selecting circuit 4, an estimated vehicle speed storingcircuit (vehicle speed storing means) 6 for storing the computedestimated vehicle speed therein as data, and a comparator (comparingmeans) 7 for comparing the estimated vehicle speed stored in theestimated vehicle speed storing circuit 6 with each follower wheelvelocity computed by the wheel speed computing circuit 3A. In this case,the output of the comparator 7 is used to control the estimated vehiclespeed computing circuit 5.

Incidentally, the follower wheel rotational speed sensor 1 and the wheelspeed computing circuit 3A serve as a first wheel speed detecting means,whereas the drive wheel rotational speed sensor 2 and the wheel speedcomputing circuit 3B serve as a second wheel speed detecting means.

The vehicle speed estimating apparatus according to the presentembodiment is basically constructed as described above. A descriptionwill now be made of a method of estimating the speed or velocity of amotorcycle, for example, which is carried out by the vehicle speedestimating apparatus.

First of all, during a period in which the motorcycle is running, thefollower wheel rotational speed sensor 1 and the drive wheel rotationalspeed sensor 2 respectively detect the rotational speeds of the frontand rear wheels as pulses outputted from a rotary encoder or the like,for example, and output same to the corresponding wheel speed computingcircuits 3A, 3B. The wheel speed computing circuits 3A, 3B compute afollower wheel speed or velocity V_(WF) and a drive wheel speed orvelocity V_(WR) in response to the pulses inputted from the followerwheel rotational speed sensor 1 and the drive wheel rotational speedsensor 2 respectively, and output the velocities thus computed to thewheel speed selecting circuit 4 as data. Incidentally, the wheelvelocities V_(WF), V_(WR) can be obtained by counting the number ofpulses outputted from the follower wheel rotational speed sensor 1 andthe drive wheel rotational speed sensor 2 and converting same into theperipheral speeds of the wheels.

The wheel speed selecting circuit 4 then selects the fastest wheelvelocity from the wheel velocities V_(WF), V_(WR) and outputs it to theestimated vehicle speed computing circuit 5 as a wheel velocity V_(WM).That this selection is made because the fastest wheel velocityapproaches an actual vehicle speed provided that a slip ratio of eachwheel against a road surface is less than or equal to 0.

The estimated vehicle speed computing circuit 5 basically computes anestimated vehicle speed or velocity V_(ref) for each given computingperiod or cycle (3 ms, for example) in the following manner.

Assuming that the motorcycle is not decelerated at -9.8 m/s² (-1G) orbelow and not accelerated at +29.4 m/s² (+3G) or more, the estimatedvehicle speed computing circuit 5 sets a lower limit decelerationG_(DEC) and an upper limit acceleration G_(ACC) as follows:

    G.sub.DEC =-9.8 m/s.sup.2                                  (1)

    G.sub.ACC =+29.4 m/s.sup.2                                 (2)

When a wheel acceleration dV_(WM)(n) /dt falls within a range of fromabove G_(DEC) to below G_(ACC), an estimated vehicle velocity V_(ref)(n)is determined as follows:

    V.sub.ref(n) =V.sub.WM(n)                                  (3)

where (n) represents a value obtained upon execution of an nthcomputation.

When the wheel acceleration dV_(WM)(n) /dt is reduced at the low limitdeceleration of G_(DEC) or below, the estimated vehicle velocityV_(ref)(n) is determined as follows:

    V.sub.ref(n) =V.sub.ref(n-1) -ΔG.sub.DEC             (4)

where V_(ref)(n-1) represents an estimated vehicle velocity in theprevious computing cycle and ΔG_(DEC) represents a lower limitdeceleration corresponding to the given computing cycle Δt, i.e.ΔG_(DEC) =G_(DEC) ×Δt. This equation (4) represents that the estimatedvehicle velocity V_(ref) is set to the lower limit deceleration G_(DEC)when the wheel velocity V_(WM)(n) is reduced at the low limitdeceleration of G_(DEC) or below.

Similarly, when the wheel acceleration dV_(WM)(n) /dt is increased atthe upper limit acceleration of G_(ACC) or above, the estimated vehiclevelocity V_(ref)(n) is determined as follows:

    V.sub.ref(n) =V.sub.ref(n-1) +ΔG.sub.ACC             (5)

where ΔG_(ACC) represents an upper limit acceleration corresponding tothe given computing cycle Δt, i.e. ΔG_(ACC) =G_(ACC) ×Δt. This equation(5) shows that the estimated vehicle velocity V_(ref) is set to theupper limit acceleration G_(ACC) when the wheel velocity V_(WM)(n) isincreased at the upper limit acceleration of G_(ACC) or above.

FIG. 5 shows the estimated vehicle velocity V_(ref) determined based onthe above equations (3), (4) and (5) when the wheel velocity V_(WM)indicated by the dashed line is given by the wheel speed selectingcircuit 4.

In the present embodiment, the estimated vehicle velocity V_(ref)determined in the above-described manner is further corrected bycomparing with the follower wheel velocity V_(WF). This process will bedescribed below with reference to FIG. 6.

FIG. 6 shows a case in which the acceleration is made by increasing adriving force of a motorcycle's engine. The velocity V_(WR) of the rearwheel, which serves as the drive wheel, abruptly increases with respectto the velocity V_(WF) of the front wheel which serves as the followerwheel because the rear wheel is brought to an idle running state uponits initial acceleration. At this time, the wheel speed selectingcircuit 4 selects the fastest wheel velocity from V_(WR), V_(WF) andoutputs the velocity V_(WR) of the drive wheel to the estimated vehiclespeed computing circuit 5 as the wheel velocity V_(WM). When, on theother hand, the estimated vehicle speed computing circuit 5 computes theestimated vehicle velocity V_(ref) based on the wheel velocity V_(WM) asit is, the estimated vehicle velocity V_(ref) different from an actualvehicle velocity is obtained.

Thus, in the present embodiment, the estimated vehicle velocityV_(ref)(n-1) obtained by the previous computation is temporarily storedin the estimated vehicle speed storing circuit 6 as data. Then, theestimated vehicle velocity V_(ref)(n-1) is compared with a followerwheel velocity V_(WF)(n) for the present computation in the comparator 7to thereby correct the estimated vehicle velocity V_(ref)(n).

That is, the comparator 7 compares the follower wheel velocity V_(WF)(n)and the estimated vehicle velocity V_(ref)(n-1). If the followingcondition is satisfied, then a signal for inhibiting the estimatevehicle velocity V_(ref) from being brought up to date is outputted tothe estimated vehicle speed computing circuit 5.

    V.sub.ref(n-1) >V.sub.WF(n) and V.sub.ref(n-1) <V.sub.WN(n)(6)

Accordingly, the estimated vehicle speed computing circuit 5 outputs anestimated vehicle velocity V_(ref)(n) identical to the previousestimated vehicle velocity V_(ref)(n-1). The estimated vehicle velocityV_(ref) thus computed is shown in FIG. 6.

When the drive wheel velocity V_(WR) is greater than the follower wheelvelocity V_(WF), the estimated vehicle velocity V_(ref) is changed so asto follow the follower wheel velocity V_(WF) without following thefastest wheel velocity. This is because the follower wheel can provideless slip on a road surface, thereby making it possible to obtain theestimated vehicle velocity V_(ref) close to the actual vehicle velocity.As a result, the estimated vehicle velocity V_(ref) can be computed withhigher accuracy.

FIG. 7 is a view illustrating the relationship between the followerwheel velocity V_(WF), the drive wheel velocity V_(WR) and the estimatedvehicle velocity V_(ref) computed in accordance with the presentembodiment at the time that the brake control system is in operation.The brake control system determines a slip ratio from the estimatedvehicle velocity V_(ref) and the wheel velocity V_(WF) or V_(WR) andadjusts a braking force so as to avoid an increase in the slip ratio,thereby effecting decelerating control. Thus, each of the wheelvelocities V_(WF), V_(WR) is of a substantially vibration type asillustrated in FIG. 7. Further, the wheel velocity V_(WM), which hasbeen selected by the wheel speed selecting circuit 4, is represented asindicated by the dashed line in FIG. 7. On the other hand, the computedestimated vehicle velocity V_(ref) is represented as indicated by thesolid line in FIG. 7 seeing it in broad perspective on the analogy ofFIG. 6. Incidentally, the estimated vehicle velocity V_(ref) is updatedagain upon its deceleration. It is therefore possible to obtain theestimated vehicle velocity V_(ref) similar to that shown in FIG. 6.

Thus, the estimated vehicle velocity V_(ref) closest to the actualvehicle velocity can be obtained. It is therefore possible to stably andreliably effect the brake control, for example. The driving force canalso be accurately controlled in addition to the appropriate brakecontrol. Further, since the control referred to above can be realizedwith an extremely simple arrangement, the capacity of a program can bereduced and a high-speed computation can be effected. It is thuspossible to execute computations at a shorter computing cycle or periodand to achieve an improvement in accuracy.

A description will now be made of a second embodiment which shows a casein which processing time can be reduced by a brake control method forcontrolling brake pressure so as to obtain the optimum braking force.

Referring to FIG. 9, reference numeral 10 designates a 2-wheeledautomotive vehicle, i.e., a motorcycle. The motorcycle 10 comprises amain body 12, a handle 14, a front wheel 16 and a rear wheel 18.

A brake control system 20 for carrying out the brake control methodaccording to the present embodiment is mounted to the motorcycle 10. Asshown in FIG. 8, the brake control system 20 is provided with anantilocking modulator 22. A pinion 26 is rotatably mounted to a d.c.motor 24 of the modulator 22 and maintained in meshing engagement with agear 28. The gear 28 is supported by a crank shaft 30 to which one endof a crank pin 34 is eccentrically coupled via a crank arm 32. Apotentiometer 38, which serves as a means for detecting the position ofan expander piston (which will be described later), is attached to theother end of the crank pin 34 via a crank arm 36.

A cam bearing 40 is rotatably mounted on the crank pin 34. The lower endof the cam bearing 40 is always pressed toward an upper limit positionunder the action of return springs 44 accommodated in a spring holder42. The expander piston 46 is brought into abutment against the upperend of the cam bearing 40 and displaced in upward and downwarddirections in response to an up-and-down movement of the cam bearing 40so as to open and close a cut valve 48.

A cut valve holder 50 having the cut valve 48 incorporated therein isprovided above the expander piston 46. A master cylinder 56 is connectedvia a passage 54 to an input port 52 of the cut valve holder 50. On theother hand, a wheel braking caliper cylinder 62 is connected via apassage 60 to an output port 58 of the cut valve holder 50. The mastercylinder 56 and the caliper cylinder 62 are interconnected with eachother via the passage 54, the modulator 22 and the passage 60. This pathis filled with oil for the hydraulic pressure. The master cylinder 56 isactuated to adjust the hydraulic pressure under the action of a brakelever 64 so as to cause the cut valve 48 to actuate the caliper cylinder62, thereby applying a braking force to a disk plate 66 attached to eachof the front wheel 16 and the rear wheel 18.

A motor controller 70 is electrically connected to the potentiometer 38and the d.c. motor 24. The motor controller 70 is also electricallyconnected to a control unit 72. The control unit 72 is provided with amemory 73. A fuzzy map in which a wheel slid ratio (λ) and a wheelacceleration/deceleration (α) are defined as inputs and caliper pressureis defined as an output, is stored in the memory 73 as data (see FIG.10). The fuzzy map has been previously created based on a slip ratio vs.membership value function, i.e., a membership function (see FIG. 11) ofa slip ratio (λ), a membership function (see FIG. 12) of anacceleration/deceleration (α), and a membership function (see FIG. 13)of caliper pressure.

Each of wheel speed sensors 74, 76 for detecting the speeds of the frontand rear wheels 16, 18 respectively, which have been attached to thecorresponding disk plates 66, is electrically connected to the controlunit 72.

The operation of the brake control system 20 constructed as describedabove will now be described in connection with the brake control methodaccording to the present embodiment.

Upon normal braking, the crank pin 34 is maintained at a predeterminedupper limit position by resilient forces of the return springs 44 so asto cause the cam bearing 40 mounted on the crank pin 34 to hold theexpander piston 46 in a forced-up state. Thus, the cut valve 48 isforced up by the expander piston 46 to thereby enable the input port 52to communicate with the output port 58.

The master cylinder 56 is then actuated by gripping the brake lever 64.Brake hydraulic pressure generated by the master cylinder 56 istransmitted to the caliper cylinder 62 through the passage 54, the inputport 52, the output port 58 and the passage 60 in that order, therebyapplying a caliper force to the disk plate 66.

When the control unit 72 then supplies a drive signal to the motorcontroller 70 to effect the brake control, the motor controller 70controls the direction and amount of rotation of the d.c. motor 24.Therefore, the pinion 26 mounted on an unillustrated rotatable shaft isrotated to turn both the gear 28 held in meshing engagement with thepinion 26 and the crank arm 32 fixedly mounted to the gear 28 via thecrank shaft 30, thereby displacing the crank pin 34 mounted to the crankarm 32 from the upper limit position to the lower limit position. Thus,the cam bearing 40 is lowered under the displacement action of the crankpin 34, so that the brake hydraulic pressure which acts on the expanderpiston 46, is added to the torque of the d.c. motor 24. Therefore, theexpander piston 46 is pressed against the cam bearing 40 so as to bepromptly lowered.

When the expander piston 46 is lowered a predetermined amount, the cutvalve 48 is seated to thereby block or cut off the communication betweenthe input port 52 and the output port 58. Thus, when the expander piston46 is further lowered singly, the volume on the output port 58 sideincreases so as to decrease the hydraulic pressure applied to thecaliper cylinder 62, thereby reducing a caliper force which is appliedto the front wheel 16, for example.

When, for example, an acceleration/deceleration (α) of the front wheel16 is detected based on the output of the wheel speed sensor 74 attachedto the disk plate 66 of the front wheel 16, a process for determining towhich one of sets expressed by the membership function shown in FIG. 12the acceleration/deceleration (α) corresponds is effected. Further, aslip ratio (λ) at this time is computed. Thereafter, a process fordetermining to which one of sets expressed by the membership functionshown in FIG. 11 the computed slip ratio (A) corresponds is effected.Next, desired caliper pressure is directly estimated from the fuzzy mapshown in FIG. 10 with the results of determination being regarded asinputs. If the slip ratio (λ) is "Zero" and theacceleration/deceleration (α) is "NB(Negative Big)", for example, then asignal indicative of control information that "Set the caliper pressureto "PM(Positive Medium)" is outputted.

Accordingly, the caliper pressure is directly estimated from the slipratio (λ) and the acceleration/deceleration (α) in the presentembodiment. Therefore, any complicated computing process and control areunnecessary and the caliper pressure can be promptly and smoothlyobtained, thereby enabling the optimum brake control. Further, since thecaliper pressure is estimated using the membership function, the brakecontrol can be smoothly performed without being affected by an abruptchange in a friction coefficient of a road surface. That is, asillustrated in FIG. 14, control based on a wheel speed curve approximateto an ideal wheel speed curve created under the experience of an expertrider can be performed. Thus, the brake control, which can ensure astable deceleration and provide less vehicle behavior as compared withthe conventional brake control, can be effected.

In the present embodiment as well, the brake control can be easilyperformed even if tire characteristics differ from each other. Acharacteristic curve of a radial tire, which is indicated by the brokenline in FIG. 15, represents that the peak of a friction coefficient (μ)exists on the low slip ratio (λ) side as compared with a characteristiccurve of a bias tire, which is indicated by the solid line in FIG. 15.In this case, it is simply necessary to move a membership function of aslip ratio (λ) from the position indicated by the solid line (bias tire)to the position indicated by the dashed line (radial tire) and to createa fuzzy map based on the membership function thus processed, as shown inFIG. 16.

When the tire characteristics are identical to each other and acharacteristic of a vehicle having excellent running stability isincluded, a membership function of a slip ratio (λ) (see the solid linein FIG. 17) is set to a position (slip ratio increasing position) movedto the right from a membership function of a slip ratio (λ) (see thedashed line in FIG. 17) at the time that a normal vehicle characteristicis included. It is therefore possible to effect an improvement inbraking performance with great ease.

Next, a system capable of accurately controlling brakes withoutincreasing the storage capacity, which will be illustrated as a thirdembodiment, will hereinafter be described in detail with reference tothe accompanying drawings. Incidentally, the same elements of structureas those employed in the second embodiment are identified by likereference numerals and their detailed description will therefore beomitted.

More specifically, the brake control system 20a according to the presentembodiment has a control unit 72 provided with a computing circuit 80 aswell as a memory 73 as shown in FIG. 18. A table in which a wheel slipratio λ and a wheel acceleration/deceleration α are defined as inputsand the amount of increase or decrease in hydraulic pressure at acaliper cylinder 62 is defined as an output, is stored as data in thememory 73 (se FIGS. 19(A) and 19(B). Further, a wheel speed sensor 74(76) attached to a disk plate 66, and a vehicleacceleration/deceleration sensor 78 are electrically connected to thecomputing circuit 80 of the control unit 72, for computing a slip ratioλ and a wheel acceleration/deceleration α.

Incidentally, the table employed in the present embodiment includes theslip ratio λ to which 64 addresses have been assigned and the wheelacceleration/deceleration α to which 256 addresses have been assigned. Aspace or interval L for setting data about the slip ratio λ or the wheelacceleration/deceleration α_(m), and the amount of data are respectivelyset in the following manner.

As shown in FIGS. 20 and 21, an interval L defined between adjacent slipratio data is set so as to increase as the absolute value of the slipratio λ is raised from a value approximate to zero. Similarly, aninterval L defined between adjacent wheel acceleration/deceleration datais set so as to increase as the absolute value of the wheelacceleration/deceleration α is raised from a value approximate to zero.That is, the braking performance and the vehicle running stability areexcellent as viewed from a relationship between the slip ratio λ and thefriction coefficient of the road surface. In addition, a number ofmemory areas are used in such a manner that high-resolution data areconcentrated on a range of 0% to 10% of the slip ratio (λ) in which aconvergent target slip ratio λT, which serves as a control target, isset and on a wheel acceleration/deceleration (α) range up to +1.0G,which is set from the standpoint of the braking performance and thevehicle running stability. When, on the other hand, the absolute valueof each of the slip ratio λ and the wheel acceleration/deceleration α islarge, low resolution data is enough and memory areas to be used are setso as to be reduced in number.

The operation of the brake control system 20a constructed as describedabove is identical to that of the brake control system according to thesecond embodiment.

In the table, the data about the amounts of increase and decrease in thecaliper pressure are set so as to be concentrated on the 0% to 10% rangeof the slip ratio λ, which serves as the control target and on the wheelacceleration/deceleration (α) range up to ±1.0G as shown in FIGS. 19(A)through 21. When the amounts of increase and decrease in brake pressurewith respect to the slip ratio λ are set at equal intervals a in thetable as shown in FIG. 22b by way of example, a control error ΔP1developed between the ideal amounts of increase and decrease in thebrake pressure and the amounts of increase and decrease in the brakepressure, which have been set in the table, is large when the slip ratiois λ₁, for example. Even if the slip ratio is λ₂ adjacent to aconvergent target slip ratio λT on the other hand, a control error ΔP2developed between the ideal amounts of increase and decrease in caliperpressure and the amounts of increase and decrease in the caliperpressure, which have been set in the table, is small, thereby enablingaccurate control. Further, since large quantities of data are set in thevicinity of a convergent target value, the width of amplitude ofvibration in the caliper pressure is also reduced quickly and the slipratio λ promptly converges on the target value. When, on the other hand,the absolute value of the slip ratio λ or the wheelacceleration/deceleration α falls within a large range, it is onlynecessary to set small quantities of data. Therefore, the storagecapacity of the entire memory can be reduced.

In the present embodiment as described above, when the amounts ofincrease and decrease in the caliper pressure are determined from theslip ratio λ and the wheel acceleration/deceleration α, the data aboutthe amounts of increase and decrease in the caliper pressure are set inthe table so as to be converged in the vicinity of the target slip ratiorepresentative of the convergent target or within the wheelacceleration/deceleration α of ±1.0G. Therefore, any variation in thecaliper pressure with respect to the target value is promptly reduced,so that the slip ratio λ converges on the target value. When theabsolute value of the slip ratio λ or the wheelacceleration/deceleration α falls within the large range, the smallquantities of data are set and the memory areas to be used are reduced.Therefore, the storage capacity of the entire memory can be reduced.

Next, a method of and a system for controlling the rate of increase inbrake pressure, which will be illustrated as a fourth embodiment, willhereinafter be described in detail with reference to the accompanyingdrawings. A motorcycle described in the fourth embodiment and the brakecontrol system are identical in structure to those according to thesecond embodiment, and their detailed description will therefore beomitted (see FIGS. 8 and 9).

The operation of a brake control system 20b will now be described belowin connection with the brake control method according to the presentembodiment.

Upon normal braking, a crank pin 34 is maintained at a predeterminedupper limit position by resilient forces of return springs 44 so as tocause a cam bearing 40 mounted on the crank pin 34 to hold an expanderpiston 46 in a forced-up state. Thus, a cut valve 48 is forced up by theexpander piston 46 to thereby enable an input port 52 to communicatewith an output port 58.

A master cylinder 56 is then actuated by gripping a brake lever 64.Brake hydraulic pressure generated by the master cylinder 56 istransmitted to a caliper cylinder 62 through a passage 54, the inputport 52, the output port 58 and a passage 60 in that order, therebyapplying a force to a disk plate 66 as a caliper force.

In order to perform antilock braking, a control unit 72 then supplies adrive signal to a motor controller 70 so as to control the direction andamount of rotation of a d.c. motor 24. Therefore, a pinion 26 mounted onan unillustrated rotatable shaft is rotated to turn both a gear 28 heldin meshing engagement with the pinion 26 and a crank arm 32, therebydisplacing the crank pin 34 mounted to the crank arm 32 from the upperlimit position to the lower limit position. Thus, the cam bearing 40 islowered under the displacement action of the crank pin 34, so that theexpander piston 46 and the cut valve 48 are lowered in the form of asingle unit. When the cut valve 48 is then seated, the input port 52 iscut off from communicating with the output port 58. Thereafter, theexpander piston 46 is further lowered singly. Consequently, the volumein the output port 58 side increases so as to decrease the hydraulicpressure which is applied to the caliper cylinder 62, thereby reducing abraking force which is applied to a front wheel 16, for example. Thus,the antilock braking is effected.

In the present embodiment, the caliper pressure increasing rate can bearbitrarily adjusted within an angular range α shown in FIG. 24 when thebraking is changed from the antilock braking to the normal braking. Thatis, as shown in FIG. 23, the crank angle of the crank pin 34 isrepeatedly changed to an angle of θ₁ and an angle of θ₂ at theircorresponding given time intervals of T₁ and T₂ about an operating angleθ (seating angle) (where θ is greater than θ₁ and less than θ₂, i.e., θ₁<θ<θ₂) of the cut valve 48. Now, the angle θ₁ is made or set to open thecut valve 48 so as to increase caliper pressure P₁. The angle θ₂ isdefined to close the cut valve 48 and further lower the expander piston46 to thereby reduce the caliper pressure P₁. Thus, the caliper pressureP₁ is substantially increased along an arbitrary target pressureincreasing rate R while a pressure increase and decrease is beingrepeated.

The angles θ₁ and θ₂ are detected by the potentiometer 38 attached tothe other end of the crank pin 34 via the crank arm 36. The detectedsignal is transmitted to the motor controller 70, which in turn drivesand controls the d.c. motor 24, thereby accurately holding the crank pin34.

In the present embodiment as described above, the substantial targetincreasing rate R of the caliper pressure P₁ is arbitrarily set withinthe angular range α by selecting the time intervals T₁, T₂ required tohold the crank pin 34 based on the their corresponding angles θ₁, θ₂.Thus, when the braking is changed from the antilock braking to thenormal braking as in the prior art, an abrupt increase (a so-calledbreakthrough) in the caliper pressure P₁ is not developed and anyvehicle behavior can be reduced as small as possible, thereby making itpossible to improve the control feeling.

Further, a modulator 22 is of a simple structure. Hence, the modulator22 can be greatly simplified in structure and made inexpensive ascompared with a conventional double structure type modulator.

Finally, a method of controlling the rate of increase in caliperpressure, which is to be illustrated as a fifth embodiment, willhereinafter be described in detail with reference to the accompanyingdrawings. A motorcycle and a brake control system described in thepresent embodiment are substantially identical in structure to thoseaccording to the fourth embodiment, and their detailed description willtherefore be omitted.

However, the brake control system 20b is provided with a cut valvemechanism 80 corresponding to the cut valve 48 employed in the fourthembodiment as shown in FIG. 25. As shown in FIGS. 26 through 28, the cutvalve mechanism 80 has a cylindrical communication hole 90 which isdefined in a cut valve holder 50 and whose diameter is reduced in theform of two steps toward the output port 58 as seen from the input port52. Portions of the communication hole 90, which have been reduced indiameter in the form of the two steps, are used as seat portions 94, 92respectively. A spheric cut valve 96 and an orifice valve 100 having anorifice 98 defined therein are inserted into the communication hole 90.The cut valve 96 is coupled to the orifice valve 100 via a coil spring102 and pressed downward by a resilient force of the coil spring 102 soas to be held in abutment against the seat portion 92. The orifice valve100 is brought into engagement with the upper surface of the input port52 by a coil spring 104 and pressed downward by a resilient force of thecoil spring 104 so as to be seated on the seat portion 94. A convexleading end 106 of the expander piston 46 is brought into abutmentagainst the cut valve 96 so as to displace the cut valve 96 in a desireddirection. Incidentally, the resilient force of the coil spring 104 isset so as to be larger than that of the coil spring 102.

Thus, the d.c. motor 24 is energized to displace the crank pin 34 so asto move the expander piston 46 in upward and downward directions,thereby controlling the cut valve mechanism 80 so as to be brought intothe following three basic states or conditions. More specifically, asshown in FIG. 26, the expander piston 46 is lowered to separate theleading end 106 of the expander piston 46 from the cut valve 96 so as toseat the cut valve 96 on the seat portion 92, thereby bringing thecommunication between the input port 52 and the output port 58 into acut-off state or condition (hereinafter called an "ABS condition"). Asshown in FIG. 27, the expander piston 46 is displaced upward from theABS condition so as to abut against the cut valve 96, thereby spacingthe cut valve 96 away from the seat portion 92. At this time, however,the cut valve 96 does not abut against the orifice valve 100 and theinput port 52 and the output port 58 are brought into a communicationcondition (hereinafter called an "ORIFICE condition") by the orifice 98in a state in which the orifice valve 100 has been seated on the seatportion 94. As illustrated in FIG. 28, the expander piston 46 is furtherdisplaced upward from the ORIFICE condition to bring the cut valve 96into abutment against the orifice valve 100 so as to separate theorifice valve 100 from the seat portion 94, thereby bringing the inputport 52 and the output port 58 into a communication state (hereinaftercalled a "NORMAL condition"). Under the ORIFICE condition, the coilspring 102 is compressed by separating the cut valve 96 from the seatportion 92, so that the orifice valve 100 is upwardly urged by theresilient force of the coil spring 102. Since, however, the resilientforce of the coil spring 104 for urging the orifice valve 100 in adownward direction is set so as to be larger than that of the coilspring 102, the orifice valve 100 is not separated from the seat portion94.

Accordingly, the three conditions can be changed over by effectingpositional control using the d.c. motor 24, i.e., controlling theposition of the expander piston 46 without regard to the difference inhydraulic pressure between the input port 52 and the output port 58.

The operation of the brake control system 20b constructed as describedabove will now be described below in connection with the brake controlmethod according to the present embodiment.

Upon normal braking, the crank pin 34 is maintained at the predeterminedupper limit position by the resilient forces of the return springs 44 soas to cause the cam bearing 40 mounted on the crank pin 34 to hold theexpander piston 46 in the forced-up state. Thus, the cut valve 96 isforced up by the expander piston 46 so as to cause the input port 52 tocommunicate with the output port 58 (see FIG. 28).

When the brake lever 64 is then gripped, the master cylinder 56 isactuated. Brake hydraulic pressure generated by the master cylinder 56is then transmitted to the caliper cylinder 62 through the passage 54,the input port 52, the output port 58 and the passage 60 in that order,thereby applying a caliper force to the disk plate 66 by caliperpressure P_(c).

On the other hand, the brake control system 20b is controlled based on aflowchart shown in FIG. 29 upon antilock control. That is, the controlunit 72 reads the velocities V_(W) of front and rear wheels based onsignals outputted from wheel speed sensors 74, 76 and reads adisplacement angle (hereinafter called a "crank angle") of the crank pin34 based on a signal outputted from the potentiometer 38 (Steps S1 andS2). The fastest one of the velocities V_(W) of the front and rearwheels is regarded as an estimated vehicle velocity V_(r). The estimatedvehicle velocity V_(r) is determined by effecting so-called highselection (Step S3). The wheel velocity V_(W) is then differentiated todetermine a wheel acceleration/deceleration α (Step S4). A slip ratio λis determined based on the estimated vehicle velocity V_(r) and thewheel velocity V_(W) (Step S5). Further, a vehicle deceleration β isdetermined from the estimated vehicle velocity V_(r) (Step S6). Adetermination (enable judgment or determination) is made as to whetheror not it is necessary to effect antilock (ABS) control based on boththe wheel acceleration/deceleration α and the slip ratio λ thusdetermined (Step S7). If the answer is determined to be Yes in Step S7,then the amounts of increase and decrease in the caliper pressure P_(c)are determined from the wheel acceleration/deceleration α the slip ratioλ using a table or the like, and a target crank angle θT is set (StepS8). Then, the target crank angle θT is corrected based on the vehicledeceleration β (Step S9). Now, a determination is made as to thecondition of control on the basis of the vehicle deceleration β, thecrank angle θ and the target crank angle θT or the like. The targetcrank angle θT is reset under breakthrough control only when it isdetermined based on the target crank angle θT that the above control isnecessary (Step S10). Thereafter, the d.c. motor 24 is controlled sothat the crank angle is brought to the target crank angle θT (Step S11).Incidentally, the breakthrough control is effected to increase thecaliper pressure at a given caliper pressure increasing rate in order toprevent a breakthrough described in the conventional example fromoccurring.

Incidentally, the vehicle deceleration control in Step S9 is made in thefollowing manner in accordance with a flowchart shown in FIG. 30. It isdetermined whether or not the vehicle deceleration β is more than orequal to a limit deceleration G_(L) (Step S15). If the answer isdetermined to be Yes in Step S15, it is then determined whether or not atarget crank angle θT_(L), of the previous loop is more than or equal toa target crank angle θT of the present loop, i.e., the caliper pressureP_(c) takes or assumes a pressure increasing direction (Step S16). Ifthe answer is determined to be Yes in Step S16, then the vehicledeceleration β is increased the limit deceleration G_(L) or more tothereby reset the target crank angle θT of the present loop to thetarget crank angle θT_(L) of the previous loop in such a manner that thevehicle stability is not made worse, i.e., the caliper pressure P_(c) isnot increased (Step S17).

A detailed description will now be made of the breakthrough control inStep S10 with reference to a flowchart shown in FIG. 31. It is firstdetermined whether or not a crank angle θ detected by the potentiometer38 is more than or equal to a predetermined angle A (Step S20). Thepredetermined angle A is defined as a crank angle made when the orificevalve 100 abuts against the cut valve 96 displaced upward by the leadingend 106 of the expander piston 46 so as to be spaced away from the seatportion 94. Incidentally, the crank angle is defined in such a mannerthat the displacement angle of the crank pin 34, which corresponds tothe upper limit position of the expander piston 46, is set to 0° and thedirection of the lower limit is made positive. That is, the crank angleθ smaller than the given angle A represents that the cut valve mechanism80 is already in the NORMAL state and hence not regarded as an object tobe subjected to the breakthrough control. Accordingly, the followingcircumstantial judgment is made only when the crank angle θ is more thanor equal to the predetermined angle A.

It is first determined whether or not the vehicle deceleration β is morethan or equal to 0.5G (Step S21). The vehicle deceleration β is normallymore than or equal to 0.5G upon braking under a high μ road such as anasphalt road whose surface is dry or the like. It is thereforedetermined that the state of the road surface is regarded as the high μroad if the vehicle deceleration β is more than or equal to 0.5G.

If the vehicle deceleration β is less than 0.5G, it is then determinedwhether or not the vehicle deceleration β is less than or equal to 0.2G(Step S22). The vehicle deceleration β is normally less than or equal to0.2G upon braking under a road surface (hereinafter called a "low μroad") of a low friction coefficient, such as an asphalt's road surfacewhich is wet or the like, or in a state (which will be called a"repetitive input") in which a brake input is repeated during a shortperiod of time. It is therefore determined that either the low μ road orthe repetitive input has been taken or selected if the vehicledeceleration β is less than or equal to 0.2G.

If it is determined that the vehicle deceleration β is less than orequal to 0.2G, then a flag is set (Step S23). It is then determinedwhether or not the target crank angle θT is less than or equal to agiven angle B (Step S24). That is, the amount of decrease in the caliperpressure P_(c) increases in the case of the low μ road. Therefore, thetarget crank angle θT is large. In the case of the repetitive input, thetarget crank angle θT is small as compared with the low μ road. Thus,the given angle B is set as a threshold value for each of the low μ roadand the repetitive input.

When the target crank angle θT is less than or equal to the given angleB, it is determined that the repetitive input is made. A breakthroughprocess corresponding to the repetitive input, which will be describedlater, is then executed (Step S25). Further, the flag is cleared (StepS26).

If, on the other hand, it is determined that the vehicle deceleration βis more than or equal to 0.5G, i.e., the high μ road has been taken, itis then judged whether or not the flag is up (set) (Step S27). If theanswer is determined to be Yes in Step S27, it is then determined inSteps S22 and S24 that the low μ road has been taken in the previousloop. It is thus determined that the high μ road has been taken in thepresent loop. That is, it is judged that each wheel has been changedover from the low μ road to the high μ road (hereinafter called a "μjump"). It is thereafter determined whether or not the target crankangle θT is less than or equal to a predetermined angle C (Step S28).Now, the predetermined angle C represents an angle at which abreakthrough occurs when the target crank angle θT is set to thepredetermined angle C or below.

When the target crank angle θT is less than or equal to thepredetermined angle C, a breakthrough process corresponding to the μjump, which will be described later, is effected (Step S29). Further,the flag is cleared (Step S30).

If the flag is down (reset) in Step S27, it is then determined that thehigh μ road has been selected. It is thereafter determined whether ornot the target crank angle θT is less than or equal to a predeterminedangle D (Step S31). Now, the predetermined angle D represents an angleat which a breakthrough is made when the target crank angle θT is set tothe predetermined angle D or below.

When the target crank angle θT is less than or equal to thepredetermined angle D, a breakthrough process corresponding to the highμ road, which will be described later, is carried out (Step S32).

The states of the high μ road, the μ jump and the repetitive input aredetected in the above-described manner. The breakthrough controlcorresponding to each of the high μ road, the μ jump and the repetitiveinput is effected in the following manner.

A description will first be made of the breakthrough controlcorresponding to the high μ road on the basis of the result of controlshown in FIG. 32. More specifically, when each brake is operated by arider, the brake pressure is transmitted to the caliper cylinder 62 fromthe master cylinder 56 via the cut valve mechanism 80 which is in theNORMAL state. Accordingly, the caliper pressure P_(c) is caused tofollow up an increase in the pressure (hereinafter called "masterpressure P_(m) ") of the master cylinder 56. Thus, when the braking ofeach wheel is made, the wheel velocity V_(W) is separated from theestimated vehicle velocity V_(r) so as to increase the slip ratio λ,thereby effecting the antilock braking. That is, the cut valve mechanism80 is brought to the ABS condition. Thereafter, the d.c. motor 24 isenergized under the control of the motor controller 70 to displace thecrank pin 34 so as to be brought to the target crank angle θT, therebymoving the expander piston 46 upward and downward so that the volume ofthe output port 58 increases or decreases. As a result, the caliperpressure P_(c) can be controlled so as to reach a predetermined pressurevalue P1 or less (see 1 FIG. 32). When the braking is changed from theantilock braking to the normal braking by returning the wheel velocityV_(W) to the velocity adjacent to the estimated vehicle velocity V_(r),the caliper pressure P_(c) gradually increases at a rate set between thepredetermined pressure value P1 and a limit pressure value P2 (see 3 inFIG. 32) after the caliper pressure P_(c) has been caused to follow upthe master pressure P_(m) up to the predetermined pressure value P1 (see2 in FIG. 32). The caliper pressure P_(c), which has reached the limitpressure value P2, is held constant as it is (see 4 in FIG. 32).

Now, the slow increase in the caliper pressure P_(c) at the rate set inthe range from the predetermined pressure value P1 to the limit pressurevalue P2 is made from the following reason. The vehicle deceleration βis computed based on the difference between an estimated vehiclevelocity V_(r) detected from a computing loop used several times beforeor several tens times before as seen from the present computing loop andan estimated vehicle velocity V_(r) detected from the present computingloop in order to eliminate noise components. Therefore, a difference isdeveloped between the vehicle deceleration β and an actual vehicledeceleration. When the pressure increasing rate is high, an increase inthe vehicle deceleration cannot be sensed before the caliper pressureP_(c) exceeds the limit pressure value P2. That is, since the routineprocedure for the vehicle deceleration control (Steps S15 through S17)cannot be executed, a rear-wheel ground load is reduced, thereby causinga risk that the running stability of the vehicle is impaired.

A description will now be made of the breakthrough control correspondingto the μ jump on the basis of the result of control shown in FIG. 33.More specifically, when each brake is operated by the rider, the brakepressure is transmitted to the caliper cylinder 62 from the mastercylinder 56 via the cut valve mechanism 80 which is in the NORMAL state.Accordingly, the caliper pressure P_(c) is caused to follow up anincrease in the master pressure P_(m) (see 1 FIG. 33). Since, however,the state of the road surface is brought to the low μ road, the wheelvelocity V_(w) is quickly reduced and the slip ratio λ increases.Therefore, the cut valve mechanism 80 is brought to the ABS conditionand the expander piston 46 is lowered to increase the volume of theoutput port 58, thereby returning the wheel velocity V_(W) to thevelocity adjacent to the estimated vehicle velocity V_(r). The expanderpiston 46 is hereafter displaced upward and downward under the ABScondition to vary the volume of the output port 58, thereby controllingthe slip ratio λ so as to fall within a predetermined slip ratio (see 2in FIG. 33). When the state of the road surface along which each wheeltravels, is changed over from the low μ road to the high μ road, agripping force of each wheel increases to make the estimated vehiclevelocity V_(r) substantially identical to the wheel velocity V_(W).Consequently, the slip ratio is reduced to thereby change over thebraking from the ABS braking to the normal braking. Accordingly, thecaliper pressure P_(c) increases while following up the master pressureP_(m). However, when the front wheel is used, the pressure increasingrate is set in such a manner that the time Δ_(t1) (between t₃ and t₄)required to change over the braking from the ABS braking to the normalbraking falls within a set time range, preferably a range from 0.1s to0.3s (see 3 in FIG. 33). This setting is made based on the followingreason. In the case of the μ jump, a time difference is developedbetween a transition of the state of the running road surface of thefront wheel from the low μ road to the high μ road and a transition ofthe state of the running road surface of the rear wheel from the low μroad to the high μ road. When the front wheel is placed under the normalbraking and the rear wheel is placed under the antilock braking during aperiod corresponding to the time difference, the difference in thebraking forces between the front and rear wheels increases and hence thecontrol feeling tends to make worse.

A description will be finally made of the breakthrough controlcorresponding to the repetitive input with reference to FIG. 34. Morespecifically, when the brake operation is repeated by the rider, thecaliper pressure P_(c) is first increased while following up the masterpressure P_(m) under the normal braking upon the first brake input (seea section I) (see 1 in FIG. 34). The slip ratio λ increases with adecrease in the wheel velocity V_(W) to thereby change the braking fromthe normal braking to the ABS braking. That is, the caliper pressureP_(c) is controlled so as to reach predetermined caliper pressure orbelow (see 2 in FIG. 34). Further, the caliper pressure P_(c) alsodecreases with a reduction in the brake input, i.e., a reduction in themaster pressure P_(m) (see 3 in FIG. 34).

When the brake input (see a section II) is then made again within agiven time interval, it is necessary that the caliper pressure P_(c) isproportional to the master pressure P_(m) (brake input). That is, thisis because the rider desires to carry out subtle braking by experiencingthe sensation of the amount of operation of each brake by the rider fromthe actual vehicle deceleration β. Accordingly, the caliper pressureincreasing rate is set in such a manner that the caliper pressure P_(c)is raised up to a given pressure value capable of providing thesensation of the actual vehicle deceleration β by the rider during agiven time interval Δ_(t2) between a brake operation time t₅ and a timet₆ at the time that a given period has passed since the brake operationtime t₅ (see 4 in FIG. 34). The time interval Δ_(t2) is preferably lessthan or equal to 0.3 ms.

Thus, the caliper pressure P_(c) is increased and controlled accordingto the set caliper pressure increasing rate (see 3 in FIG. 32, 3 in FIG.33 and 4 in FIG. 34). As shown in FIG. 35 by way of example, the targetcrank angle θT is set to each of a crank angle G for bringing the cutvalve mechanism 80 to the ORIFICE condition and crank angles H, I forbringing the cut valve mechanism 80 to the ABS condition, so as to beassociated with a crank angle E at which the leading end 106 of theexpander piston 46 abuts against the cut valve 96 and a crank angle F atwhich the cut valve 96 abuts against the orifice valve 100. Based on thecrank angles G, H, I, the motor controller 70 is activated to energizethe d.c. motor 24. Accordingly, the crank pin 34 is displaced based onthe target crank angle θT to move the expander piston 46 in the upwardand downward directions so as to repeatedly seat and separate the cutvalve 96 on and from the seat portion 92. Thus, when the cut valvemechanism 80 is in the ORIFICE condition, it is activated to transmitthe master pressure P_(m) from the input port 52 to the output port 58via the orifice 98, thereby increasing the caliper pressure P_(c) at thecaliper pressure increasing rate shown in FIG. 36. This caliper pressureincreasing rate can be set to a desired caliper pressure increasing rateby suitably changing a target pattern.

In the present embodiment as described above, the condition of a roadsurface is estimated from a vehicle deceleration β. The state of a brakeoperation is detected based on a crank angle θ and a target crank angleθT. The increasing rate of caliper pressure P_(c) at the time that thebraking is changed from the antilock braking to the normal braking, isset according to the conditions of both the road surface and the brakeoperation. A target crank angle θT corresponding to each of ORIFICE andABS conditions is set based on a given pattern so as to meet the caliperpressure increasing rate. A d.c. motor 24 is then energized based on thetarget crank angle θT. Accordingly, the brakes can be applied on avehicle at the caliper pressure increasing rate corresponding to theconditions of both the road surface and the brake operation, therebymaking it possible to improve the control feeling or the like.

According to a vehicle velocity estimating method and a vehicle velocityestimating apparatus of the present invention, as has been describedabove, a high-accuracy estimated vehicle velocity corresponding to anactual vehicle velocity can be obtained, thereby making it possible tocontrol brakes and driving forces, for example, with high accuracy usingthe estimated vehicle velocity thus obtained. Further, the controlitself is easy and hence the entire structure is also simple. As aresult, a high-speed computation can be effected by using a simpleprogram. Accordingly, the number of computations can be increased,thereby making it possible to achieve a further improvement in accuracy.

In a brake control method according to the present invention, after awheel slip ratio and a wheel acceleration/deceleration have beendetermined, a target braking force for each brake can be directlyestimated based on a membership function in which the wheel slip ratioand the wheel acceleration/deceleration are defined as inputs.Therefore, complex control is unnecessary and the optimum brake controlcan be effected by a simple process.

The amounts of increase and decrease in caliper pressure are set in atable stored as information in a storing means of a brake control systemaccording to the present invention so as to be associated with the valueof a given wheel acceleration/deceleration velocity and the value of aslip ratio. However, the value of the wheel acceleration/decelerationand the value of the slip ratio are set so as to be brought into highresolution in the vicinity of convergent target values for the wheelacceleration/deceleration and the slip ratio. Therefore, the convergencyof the caliper pressure with respect to the convergent target values isimproved. Further, the storage capacity can be reduced because thevalues of the wheel acceleration/deceleration and the slip ratio are setso as to be brought into low resolution as the brake pressure isseparated from the convergent target values.

Further, in the brake control method and the brake control systemaccording to the present invention, the opening and closing of a cutvalve is repeatedly carried out at given time intervals under anup-and-down movement of an expander piston. Therefore, a caliperpressure increases upon the opening of the cut valve and decreases uponthe closing of the cut valve. Accordingly, the caliper pressure can beincreased along an arbitrary target pressure increasing rate byadjusting time intervals required to open and close the cut valve,thereby making it possible to prevent an abrupt increase in the caliperpressure and to improve the controllability. Further, a modulator, whichis simple in structure, is available and inexpensive.

Furthermore, the state of a road surface and the state of input of abrake operation are detected. Then, the optimum pressure increasing rateof a caliper cylinder is set based on the detected states. The expanderpiston can be displaced in accordance with the optimum pressureincreasing rate so as to increase caliper pressure, thereby making itpossible to reliably achieve a further improvement in the controlfeeling or the like.

Having now fully described the invention, it will be apparent to thoseskilled in the art that many changes and modifications can be madewithout departing from the spirit or scope of the invention as set forthherein.

What is claimed is:
 1. A method of estimating a vehicle velocity, said method comprising the following steps:a first step of determining velocities of drive wheels and follower wheels of said vehicle; a second step of selecting the fastest velocity of said velocities determined in said first step; a third step of determining a current estimated velocity V_(ref)(n) of said vehicle based on said fastest wheel velocity selected in said second step; and a fourth step of repeatedly executing said first through third steps at given time intervals through a (n) and setting the estimated vehicle velocity determined in a previous (n-1) cycle as said current estimated vehicle velocity when said estimated vehicle velocity determined in said third step is faster than said follower wheel velocities determined in said first step and lower than said fastest wheel velocity selected in said second step, wherein the estimated velocity V_(ref)(n) is determined in said third step as follows:(i) if said vehicle has not decelerated below a predetermined lower limit deceleration G_(DEC) or accelerated above a predetermined upper limit acceleration G_(ACC), then

    V.sub.ref(n) =V.sub.WM(n)

where V_(WM)(n) is a fastest wheel velocity of said drive wheels and said follower wheels, (ii) if said vehicle has decelerated below said lower limit deceleration, then

    V.sub.ref(n) =V.sub.ref(n-1) -ΔG.sub.DEC

where V_(ref)(n-1) is an estimated vehicle velocity determined in the previous cycle (n-1), and ΔG_(DEC) is a lower limit of a deceleration in a current cycle, and (iii) if said vehicle has accelerated above said upper limit acceleration, then

    V.sub.ref(n) =V.sub.ref(n-1) +ΔG.sub.ACC

wherein ΔG_(ACC) is an upper limit of an acceleration in said current cycle.
 2. An apparatus for estimating a vehicle velocity, said apparatus comprising:first wheel velocity detecting means for detecting velocities of drive wheels; second wheel velocity detecting means for detecting velocities of follower wheels; wheel velocity selecting means for selecting a fastest velocity of respective wheel velocities detected by said first and second wheel velocity detecting means; vehicle velocity estimating means for estimating the velocity of said vehicle based on said fastest wheel velocity selected by said wheel velocity selecting means, and for periodically updating the estimated vehicle velocity through a plurality of cycles (n), said vehicle velocity estimating means comprising: circuit means for setting a predetermined lower limit deceleration G_(DEC) and a predetermined upper limit acceleration G_(ACC), and for periodically determining a current estimated vehicle velocity V_(ref)(n) as follows:(i) if said vehicle has not decelerated below said predetermined lower limit deceleration G_(DEC) or accelerated above said predetermined upper limit acceleration G_(ACC), then

    V.sub.ref(n) =V.sub.WM(n)

where V_(WM)(n) is a fastest wheel velocity of said drive wheels and said follower wheels, (ii) if said vehicle has decelerated below said lower limit deceleration, then

    V.sub.ref(n) =V.sub.ref(n-1) -ΔG.sub.DEC

where V_(ref)(n-1) is an estimated vehicle velocity determined in a previous cycle (n-1), and ΔG_(DEC) is a lower limit of a deceleration in a current cycle, and (iii) if said vehicle has accelerated above said limit acceleration, then

    V.sub.ref(n) =V.sub.ref(n-1) +ΔG.sub.ACC

wherein ΔG_(ACC) is an upper limit of an acceleration in said current cycle; vehicle velocity storing means for storing therein said current estimated vehicle velocity determined by said vehicle velocity estimating means; and comparing means for comparing said vehicle velocity stored in said vehicle velocity storing means and said follower wheel velocities detected by said second wheel velocity detecting means; wherein said vehicle velocity estimating means is activated to suspend updating of said estimated vehicle velocity when it is determined based on said result of comparison by said comparing means that said vehicle velocity is faster than the velocities of said follower wheels. 